The invention is directed to a navigational system for automatically positioning a platform relative to the earth's geopotential surface. Such a navigational system has many applications in controlling the orientation of an object such as, for example, an aircraft or satellite or other vehicle. For example, in satellite control, it is important that an orbiting satellite include an attitude stabilization system which is oriented relative to the earth. Although efforts have been made to provide such stabilization systems, this invention provides improved means to effect accurate attitudinal reference with respect to what is referred to herein as the "true vertical." By "true vertical" is meant the perpendicular to the geopotential surface of the earth. Such an accurate attitudinal reference is required in order to properly align and/or stabilize at least one axis of the satellite relative to the earth.
The invention described herein also finds use in those navigational systems wherein accelerometers are used to effect leveling of a platform relative to one or more axis. In such systems it is necessary to ascertain the orientation of each accelerometer relative to the true vertical. This requirement is obviously important in the positioning of the accelerometer relative to, or the correction of the output signals which it generates as a function of, the true vertical. In those instances where the accelerometer is not truly vertical, its lack of proper orientation causes undesirable oscillations in the Schuler tuner loops ordinarily utilized in such systems, thereby causing navigational errors. Prior efforts to reduce the magnitude of such oscillations include the use of well known doppler radar units. However, such units are not always effective, and particularly in applications requiring a self-contained non-transmitting system the doppler radar aided mechanization is undesirable. To overcome the shortcomings of the doppler radar units, the present invention provides an accurate self-contained attitudinal reference to true vertical and may be used for damping an inertial navigation system.
Some prior art positioning systems utilizing accelerometer data to position a moving platform relative to the localized vertical are effective in correcting translational errors caused by movement. However, such systems do not correct for non-alignment of the platform caused by gravitational anomalies and deflections of the vertical which result therefrom. Such gravitational anomalies are inherent as a result of the inhomogenuity of the mass of the earth. Similar systems have relied upon geomagnetic data in addition to accelerometer data as positioning means. However, such systems require use in those environments where geomagnetic anomalies are not important since it is clear that geomagnetic anomalies are often at least as detrimental, if not more so, than gravitational anomalies.
Accordingly, it is an object of this invention to provide a navigational system which references a platform to the earth as desired by correcting (a) errors caused by motion of the platform, and (b) errors caused by gravitational anomalies caused by the inhomogenuity of the earth's surface, and the resulting deflections of the vertical.
Another object of this invention is to provide a navigational system having a gimballed platform to which may be mounted any navigation instrument.
Still another object of this invention is to provide a navigational system which is not subject to the problem of geomagnetic anomalies.
A further object of this invention is to provide a navigational system which is not subject to the problem of gravitational anomalies.
Still a further object of this invention is to provide a gravity gradient level which automatically provides accurate and dynamic alignment of a platform relative to the geopotential surface of the earth irregardless of curvature variations in the geopotential along the path of travel of the platform.
Yet another object of this invention is to provide a navigational system which is capable of self-levelling in free flight and operates effectively in orbit.
Another object of this invention is to provide a navigational system which effects accurate attitudinal reference with respect to the true vertical.
A further object of this invention is to provide a navigational system wherein an inertial platform can be automatically rotated into a datum reference spheroid.
Another object of this invention is to provide a navigational system wherein a level platform is aligned with the true vertical to the geopotential surface of the earth.